The present invention relates to measurement of vibrations of objects based on reflectance of speckle interference patterns from incident waves, and in particular embodiments, to optically based in vivo measurement of the small amplitude vibrations of the bones of the middle ear.
Measurement of the displacement or velocity of the bones in the middle ear is one way to objectively characterize the function of the tympanic membrane and ossicular chain, as well as the stapedial footplate. This measurement has been done using a variety of physical measurements. Gyo et al., "Measurement of the Ossicular Vibration Ratio in Human Temporal Bones by Use of a Video Measuring System", Acta Otolaryngol (Stockh) 103, 87-95, 1987, disclose a video measuring system to determine the vibration of the ossicles in a human temporal bone resulting from a sound stimulus. While this procedure is able to measure displacement amplitudes down to 0.3 .mu.m, it suffers from being extremely expensive to implement and requires the placement on the ossicular chain of a bead that can be imaged.
Von Unge et al., "Displacement of the Gerbil Tympanic Membrane Under Static Pressure Variations Measured With a Real-time Differential Moire Interferometer", Hearing Res. 70, 229-242, 1993, disclose a real-time differential moire interferometer to measure tympanic membrane displacement in a gerbil model. This method allows one to image the actual shape of the tympanic membrane under different pressure conditions, but it again is expensive and very difficult to implement because of the requirement of real-time image analysis for extraction of quantitative data. Others have used heterodyne Michelson interferometry to extract sub-micron movements; e.g., Kossl et al., "Basilar Membrane Resonance in the Cochlea of the Mustached Bat", Proc. Natl. Acad. Sci. USA, 92, 276-279, 1995; and Decraemer et al., "A Method for Determining Three-Dimensional Vibration in the Ear", Hearing Res. 77, 19-37, 1994. However, it is unlikely such a method could be economically used in vivo due to variations in the index-of-refraction of the atmosphere in each arm of the interferometer which can introduce spurious readings. Also, analysis of the output from an interferometer requires complex calculation steps to obtain movement information from fringe count.
Finally, Goode et al., "Measurement of Umbo Vibration in Human Subjects-Method and Possible Clinical Applications", Am.J.Otol. 14,247-251, 1993, disclose an extremely sensitive technique to measure umbo vibration in vivo using a laser doppler vibrometer.
This instrument has produced some very interesting measurements, such as the peak-to-peak displacement of the umbo and stapes footplate of 0.002 to 0.7 .mu.m when 105 dB SPL pure tone impinges on the TM. More recently, this instrument (Polytec PI Inc., CA) has been used to measure umbo velocity in the rat. While this instrument has the benefits of providing a (potentially) non-contact measurement, and a wide frequency and amplitude range with high spatial resolution, its cost is prohibitive in today's medical environment ($30,000-$40,000) and in practice, it and other instruments like it, require affixing a small reflective object on the object of interest, which has the further disadvantage of affecting the vibrational characteristics of the object being measured.
U.S. Pat. No. 5,394,233 discusses several variations on the basic doppler approach which dominates laser vibrometer technology. Some vibrometers exploit a difference between an incident and reflected laser beam by relying on mirrors or corner reflectors attached to the vibrating object. Examples of devices based on this concept are disclosed in U.S. Pat. Nos. 4,768,381 and 4,185,503.
It is desirable to be able to monitor the success of middle ear surgery such as when prostheses are implanted. Measurement of the reaction of the implanted prostheses to sound indicates the success of the procedure by objective measurement without closure of the wound and without depending on subjective reactions of the patient. Such a device could also be employed to monitor hair cell motion in the cochlea or tympanic membrane displacement. It is also highly desirable that such a device be suitable for use in vivo.
It has not been recognized that the surface of the bones of the middle ear and prostheses, provides sufficient reflectivity, both diffuse and specular (mirror-like), to allow direct optically based vibration measurements based on detection of the reflected beam. The prior art has relied on specular reflectance and, as mentioned above, has resorted to expedients such as the attachment of a mirror-like object on the object of interest in order to obtain the necessary degree of reflectance.
In order to overcome the problems and limitations of the prior art, it is therefore desirable to develop an inexpensive ergonometrically designed instrument to objectively and scientifically measure the vibrational amplitude transmitted by vibrating objects, and in particular, the bones in the middle ear. Such an instrument would, for example, allow a surgeon to objectively test the quality of a bone prosthesis surgery in vivo immediately after surgery, but before closing the operative field. The benefits of such an instrument include providing an objective measurement of acoustic transmission and precluding the need for immediate corrective surgery.
An instrument suitable for measuring the acoustic signal produced by the vibration of bones and membranes should exhibit these features: it should be inexpensive compared to the prior art, it should be portable, it is desirable that it run on rechargeable batteries, it should be usable in conjunction with millimeter-sized endoscopes, it should be hand-held and easy to use, it should be visually directed to interrogate millimeter-sized objects, it should not be affected by ambient and unrelated acoustic signals.